Catalytic Sonochemistry for Clean Hydrogen from Ammonia

The UK plans to undergo a "green industrial revolution" to mitigate global warming and reach net-zero by 2050. Switching to hydrogen, a promising zero-carbon fuel, is part of this plan and requires a massive improvement on the current hydrogen economy and associated technologies. Hydrogen gas, however, is difficult to store and transport, limiting its utility. It is desirable to chemically store hydrogen in ammonia because it is safer and easier to contain and transport and benefits from an established supply chain. However, cracking ammonia back to hydrogen requires catalysts that delicately balances two rate-limiting steps that inhibit the reaction: 1) rapid desorption of ammonia from the catalyst at hot temperatures and 2) inability to reform hydrogen and nitrogen from ammonia bound to the catalyst at cold temperatures. Under fixed operating conditions, this balance creates an optimal temperature for catalyst activity only achieved with rare and expensive elements operating at high temperatures, thus challenging the utility of ammonia as a hydrogen store.

Interestingly, this theoretical maximum for static catalysis may be overcome by rapidly switching between operating conditions that favour these opposing rate-limiting steps, i.e., dynamic catalysis. For ammonia cracking, this involves shifting between cold (< 150 C) and hot (> 500 C) temperatures a thousand to a million times per second, which is operationally difficult.

Sonochemistry uses sound to create bubbles that expand and contract to enhance chemical reactions and may provide a unique means of rapidly oscillating temperature. As a bubble expands and contracts above its initial size, its temperature remains equal to the ambient temperature whereby ammonia will adsorb onto the catalyst. Below its initial size, the bubble may rapidly shrink, compressing the gas and causing it to heat up to temperatures above 500 C. These hot compressions create a local high-energy microenvironment ideal for catalytic cracking of ammonia. After compression, the bubble expands back to its original size, cooling back to ambient temperatures and starting the cycle again.

This approach to sonochemistry requires site-controlled bubble motion around a catalyst. Yet current sonochemical processes do not control bubble dynamics. We have recently shown that nanostructured catalysts that also function as nucleation sites for bubbles vastly improve reaction rates. However, this work used simpler chemistry as a proof-of-concept and did not fully exploit the potential in addressing more challenging heterogenous catalytic reactions.

This project seeks to advance our approach to sonochemistry to achieve ammonia cracking. We hypothesize that rapid hot-cold cycles are achievable with bubbles nucleated by nanostructured catalysts and will overcome the conventional kinetic limitations associated with ammonia cracking. We start the project by first developing novel catalytic cavitation agents and study their sonochemistry using simpler chemistries. After, we will advance cavitation metrology and demonstrate that ammonia cracking is possible. These results will then be used in technoeconomic models to assess the potential industrial impact. Our key novelty is the combination of cavitation agents with catalysts to enhance sonochemical processes, which has yet to be done and is a paradigm shift in sonochemistry. As such, Shell, ExxonMobil, NPL, and SENFI UK Ltd. all support our research vision, proposed project, and desire to achieve a sustainable route to clean hydrogen production.